2013年9月29日（周日）上午10:30，物理系seminar

報告題目： Single molecule study of transcription under torsion 報告人： Jie Ma Physics department, Cornell University，USA 報告時間： 2013-9-29 10:30 報告地點：理科樓三樓報告廳摘要： RNA polymerases (RNAPs) are motor enzymes that carry out transcription, a critical step in gene expression and regulation. During transcription elongation, RNAP tracks the helical groove of DNA, generating (+) DNA supercoiling in front of itself, and (-) DNA supercoiling behind. However, the impact of DNA supercoiling on transcription and gene regulation is not well understood. Here, we report direct measurements of transcription under torsion. Using novel angular optical trap-based assays, we monitored the movement of an individual E. coli RNAP as it introduced and worked against either (+) DNA supercoiling downstream or (-) DNA supercoiling upstream. We found that RNAP stalled at a mean torque of 11 ± 4 pN·nm, significantly higher than a lower bound previously estimated. RNAP is thus a powerful torsional motor, capable of generating sufficient torque to change DNA topology and structure (e.g., melt DNA). Upon torque relaxation, ~ 50% of stalled RNAPs resumed transcription within 90 s. This suggests that a significant fraction of stalled transcription complexes in vivo are able to recover if torsional stress is relaxed either by topoisomerases or through DNA rotation. Furthermore, when RNAP was subjected to a short pulse of torque, well beyond the stall torque, a majority of RNAPs continued transcription immediately after the pulse, suggesting that RNAP is resilient to transient torque fluctuations. Additionally, we characterized the torque-velocity relationship. A resisting torque slowed the forward translocation rate, and increased pause frequency and duration. Together, these results provide a quantitative framework for understanding how DNA supercoiling regulates gene expression. 個人簡介： Jie Ma earned his B.S. degree in Physics (2000) at Tsinghua University, China. He got his Master degree in Electrical Engineering (2003) also at Tsinghua University under supervision of Prof. Yi-kang Pu. Later, he went to University of Bristol in U.K. and completed his PhD study in School of Chemistry (2008) under supervision of Profs. Mike Ashfold and Andrew Orr-Ewing. His PhD work is about using highly sensitive laser spectroscopy (such as CRDS and TDLAS) to explore the gas-phase chemistry in diamond CVD. After graduation, he stayed in the same lab for about one year as an EPSRC Postdoc. In 2009, he was hired by Howard Hughes Medical Institute (HHMI) and moved to Cornell University to study single molecule biophysics under supervision of Prof. Michelle Wang. He is now an HHMI Postdoctoral Research Associate. His research interest includes single molecule biophysics, optics, laser spectroscopy, plasma physics and chemistry. He has published more than 15 papers.